The divisions of the day which different nations have employed, are denoted by hour-lines of various kinds on the sphere. Of these hour-lines, drawn on a supposition that neglects the inequalities of the Earth's motion, there are three kinds.
The first kind denotes hours counted from the meridian, equal to each other at all declinations of the sun. These lines are great circles on the sphere, passing through the poles of the equator, and every pair intercepting a similar arc on each of the parallels. Of this kind are the hour-lines of sidereal time, counted from the meridian, and the hour-lines of solar time, counted from the meridian.